Colon

The colon ( : ) is a punctuation mark consisting of two equally sized dots centered on the same vertical line. A colon precedes an explanation or an enumeration, or list. A colon is also used with ratios, titles and subtitles of books, city and publisher in bibliographies, biblical citations between chapter and verse, and for salutations in business letters and other formal letter writing, and often to separate hours and minutes.

Usage

The most common use of the colon is to inform the reader that what follows the colon proves, explains, defines, describes, or lists elements of what preceded it. In modern English usage, a complete sentence precedes a colon, while a list, description, explanation, or definition follows it. The elements which follow the colon may or may not be a complete sentence: since the colon is preceded by a sentence, it is a complete sentence whether what follows the colon is another sentence or not. While it is acceptable to capitalize the first letter after the colon in American English, it is not the case in British English, except where a proper noun immediately follows a colon.

Colon used before list

Williams was so hungry he ate everything in the house: chips, cold pizza, pretzels and dip, hot dogs, peanut butter, and candy.

Colon used before a description

Jane is so desperate that she'll date anyone, even Tom: he's uglier than a squashed toad on the highway, and that's on his good days.

Colon before definition

For years while I was reading Shakespeare's Othello and criticism on it, I had to constantly look up the word "egregious" since the villain uses that word: outstandingly bad or shocking.

Colon before explanation

I had a rough weekend: I had chest pain and spent all Saturday and Sunday in the emergency room.

Some writers use fragments (incomplete sentences) before a colon for emphasis or stylistic preferences (to show a character's voice in literature), as in this example:

Dinner: chips and juice. What a well-rounded diet I have.

Syntactical-Deductive

The colon introduces the logical consequence, or effect, of a fact stated before.

There was only one possible explanation: the train had never arrived.

Syntactical-Descriptive

In this sense the colon introduces a description; in particular, it makes explicit the elements of a set.

I have three sisters: Daphne, Rose, and Suzanne.

Syntactical-descriptive colons may separate the numbers indicating hours, minutes, and seconds in abbreviated measures of time.

The concert begins at 21:45.
The rocket launched at 09:15:05.

British English, however, more frequently uses a point for this purpose:

The programme will begin at 8.00 pm.
You will need to arrive by 14.30.

A colon is also used in the descriptive location of a book verse if the book is divided into verses, such as in the Bible or the Quran:

"Isaiah 42:8"
"Deuteronomy 32:39"
"Quran 10:5"

Appositive

Luruns could not speak: he was drunk.

An appositive colon also separates the subtitle of a work from its principal title. Dillon has noted the impact of colons on scholarly articles, but the reliability of colons as a predictor of quality or impact has also been challenged. In titles, neither needs to be a complete sentence as titles do not represent expository writing:

Star Wars Episode VI: Return of the Jedi

Segmental

Like a dash or quotation mark, a segmental colon introduces speech. The segmental function was once a common means of indicating an unmarked quotation on the same line. The following example is from the grammar book The King's English:

Benjamin Franklin proclaimed the virtue of frugality: A penny saved is a penny earned.

This form is still used in written dialogues, such as in a play. The colon indicates that the words following an individual's name are spoken by that individual.

Patient: Doctor, I feel like a pair of curtains.
Doctor: Pull yourself together!

Use of Capitals

Use of capitalization or lower-case after a colon varies. In British English, the word following the colon is in lower case unless it is normally capitalized for some other reason, as with proper nouns and acronyms. British English also capitalizes a new sentence introduced by colon's segmental use; American English goes further and permits writers to similarly capitalize the first word of any independent clause following a colon. This follows the guidelines of some modern American style guides, including those published by the Associated Press and the Modern Language Association. The Chicago Manual of Style, however, requires capitalization only when the colon introduces a direct quotation, a direct question, or two or more complete sentences.

Spacing

In print, a thin space was traditionally placed before a colon and a thick space after it. In modern English-language printing, no space is placed before a colon and a single space is placed after it.

One or two spaces may be and have been used after a colon. The older convention (designed to be used by monospaced fonts) was to use two spaces after a colon.